1. Field of the Invention (Technical Field)
The present invention is a method and apparatus for making a reflection hologram (or volume hologram) that is made using a transmission hologram (or plane hologram) as the object. This invention obtains the benefits of the transmission hologram's diffraction spectral color playback and color separation of a white light or other multi-wavelength source by directly converting the transmission hologram to a reflection hologram for security and forgery prevention. The present invention preferably utilizes a single color laser, or optionally a tunable laser or other coherence light source, to record the hologram by scanning the plane hologram with a profiled narrow beam. The resulting hybrid reflection hologram, when illuminated by white light, can replay in a single color or in the multiplicity of colors of the original transmission hologram while adding the unique optical characteristics of the reflection hologram.
2. Background Art
Note that the following discussion refers to a number of publications and references. Discussion of such publications herein is given for more complete background of the scientific principles and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Differentiation of Transmission and Reflection Holograms
When constructing a hologram, as the angle difference between the object beam (or the wavefronts bouncing off the object) and the reference beam changes, so does the spacing of the patterns in the emulsion. As long as the angle difference remains less than about 90 degrees the hologram is typically called a transmission hologram, where “plane” typically means that the holographic information is primarily contained in the two-dimensional plane of the emulsion. Although the emulsion does have a thickness, typically around seven microns, the spacing between fringes is large enough, when the angle is less than about 90 degrees, so that the depth of the emulsion isn't being utilized in the recording of the hologram. At about 90 degrees, which is really a convenient but arbitrary point, the angle is great enough, and fringe spacing has become small enough, so that the recording process is taking place throughout the volume of the thickness of the emulsion, thus producing a reflection hologram. Thus the same emulsion can be used to make both transmission and reflection holograms depending on the angle difference between the reference and object beams. (However, some emulsions or other photosensitive materials are better for either transmission holograms or reflection holograms.) Thus, as the incidence angle of the reference beam is rotated, either a transmission or reflection hologram is constructed, as shown in FIG. 1.
A very important point for differentiation occurs as the reference beam swings around its arc of possible positions. In a plane (transmission) hologram the reference beam is hitting the film from the same side as the object beam. In a volume (reflection) hologram the reference beam hits the film from the side opposite to the modulated object beam. When a difference of 180 degrees is reached, an in-line, volume reflection hologram is constructed.
A transmission type hologram means that the reference beam must be transmitted through the hologram, in order to decode the interference patterns and render the reconstructed image. The light which is used for playback of a transmission hologram must be coherent or semi-coherent or the image will not be sharp. If a non-coherent source is used, such as the light from a common, unfiltered slide projector, then the hologram will diffract all the different wavelengths. The interference pattern or grating etched in the emulsion is not particular as to which wavelengths it bends or focuses; therefore, the result is an unclear overlapping spectrum of colors which somewhat resemble the object. A hologram will playback just as well with laser light of a different color or wavelength than the light with which it was made. However, the object will appear to be of a different size and/or distance from the plate. For example, a hologram of an object made with red light will playback that object smaller or seemingly further away if a blue colored laser is used to view it. This is because the grating will bend the blue or shorter light less severely than the red with which it was made and with which it is intended to be decoded.
Unlike a transmission hologram, also called a thin, transmission, laser-illuminated, or phase hologram, which requires a coherent or highly filtered playback source, a reflection hologram, also called a volume or thick hologram, can be viewed very satisfactorily in white light or light which contains many different wavelengths. For best results, the light preferably should be from a point source and have limited divergence, such as light from a slide projector light or penlight, or the sun on a clear day. Any ambient light may alternatively be used, but this will typically produce lesser quality of the playback image. This ability to use white light occurs because, in a way, a reflection hologram acts as its own filter. In a reflection hologram the fringes are packed so closely together that they constitute layers throughout the thickness of the emulsion. The spacing between fringes remains constant. If the distance between fringes is two microns, for example, then the distance between the remaining layers of fringes will also be two microns. This distance is a function of the wavelength of light used in constructing the hologram and also the angle difference between reference and object beam. This layered structure allows the reflection hologram to absorb, i.e. not reflect, any of the colors of light which do not have the correct wavelength. The wavelength which matches the fringe spacing will be reflected: the crests of the wavelengths which are too short or too long will eventually miss one of the planes and be absorbed into the darkness of the emulsion.
In a reflection type hologram the playback light or reconstruction beam comes from the same side of the hologram as the viewer. Some parts of the incident light are reflected, some are not, depending on the interference pattern. If the hologram was made correctly the result should be a visible three dimensional image. In contrast, for transmission holograms the reconstruction beam must pass through the hologram and come towards the viewer from the opposite side of the hologram. Just as very few transmission holograms are made in-line (or at 0 degrees), very few reflection holograms are made inline; otherwise the viewer would have to hold the playback light source close to his or her eyes. Most reflection holograms are made at a less severe angle, perhaps 160 degrees, so that the light can come in at an angle without being blocked by the person who is trying to see the hologram.
Real and Virtual Images
The image produced by the hologram can either appear to be in front of the holographic plate or film, or behind the film (or any position in between). As shown in FIG. 2, in the former case it is called a real image (projection); the latter is called a virtual image. In general it is easier to view a virtual image because you can see through the hologram as if it were a window. Note that the size of the window does not affect the apparent size of the image; a smaller window would simply allow a more confined view, or fewer possible angles of view, of the image. To view a virtual image the viewer looks through the hologram to perceive the object floating in the space behind it. In contrast, a real image appears in free space in front of the hologram. It is a little more difficult to view a real image because the viewer must find the image and focus his or her eyes in front of the hologram; the hologram itself is typically less capable to act as a guide for the viewer's eyes.
The real image is very exciting but there are a number of drawbacks. The object holographed should be quite a bit smaller than the size of the film you are using, or the viewer will not be able to see the complete real image of the object all at once. Also, without special precautions taken when constructing the hologram, the real image will be pseudoscopic. This means that everything that was closer to the film when the hologram was made will now be further away, and vice versa. This includes both individual objects in a shot or the different planes of space of an individual object. The pseudoscopic image is made by reversing the direction of the reference beam, or by turning the completed hologram around until seeing the image in front of the plate.
For example, referring to FIG. 3, if in making a hologram a salt shaker is placed closer to the film than a pepper shaker (the salt shaker may even cast a shadow from the object beam onto the pepper shaker), then in a pseudoscopic playback as a real image the pepper shaker will appear to be closer to the viewer than the salt shaker, which may no longer appear. However, in a virtual image of the same hologram the shakers would resume their original positions.
Image Plane Holograms
Image plane holograms are transmission holograms which are viewable in white light and made using a first, master hologram as the object for making a final, second transmission hologram. However, although the master hologram is reproduced using an open aperture, the image is achromatic (black and white), and this method can only produce extremely shallow holograms without substantial blurriness. Rainbow or Benton holograms are modified image plane holograms in which the final transmission hologram is produced using a limited aperture. This reduces blurring of deeper holograms. However, Benton holograms may only be viewed from a small angular range due to the limited aperture; the entire hologram cannot be viewed from, for example, above or below. Because a Benton hologram is a transmission hologram, color control is limited. That is, all of the colors in a Benton hologram will shift throughout the color spectrum of the viewing light source, for example white light, when the viewing angle changes.